Entamoeba histolytica is an endo-parasite and it causes two main diseases in humans, these are, amoebic dysentery and liver abscess.
It has two morphological forms i.e, trophozoite form and cyst form. trophozoite is feeding, growing, invading and disease causing form of E. histolytica and it is present inside the body, while cyst form is infective form of E. histolytica that is present in external environment at contaminated sites. It release trophozoites when it reaches intestine through oral route.
Morphology of trophozoites
Trophozoite doesn’t has a fixed shape, its shape constantly changes due to constantly changing position. Its cytoplasm is granular and may contain red cells. It moves slowly and it has special structure called pseudopodia that help in its movement. It is found in diarrheal stool, intestine and extra-intestinal lesions e.g., liver abscess.
Morphology of E. histolytica cysts.
Cysts are rounded, and it is surrounded by a highly resistible cyst wall. In early stages the cytoplasm of cyst contains a glycogen vacuole and chromotoidal bodies. These structures disappear latter when the nuclear material inside the cyst divides. Cyst has on nucleus in early stages, but soon it divides by binary fission into binucleate and quadrinucleate bodies. Unlike trophozoites these cyst are found in non-diarrheal stools, and may be in an asymptomatic patient.
Where Entamoeba histolytica lives in human body?
Entamoeba histolytica lives in sub-mucous layer of large intestine of the human body. It is found in the intestine in trophozoite form. While cysts of E. histolytica are excreted in feces and are found in external environment at contaminated sites.
How does Entamoeba histolytica enter human body?
Entamoeba histolytica is transmitted from one person to another via oral-fecal route. Usually cysts are ingested orally that cause disease. Fecal-oral route of transmission is more obvious among homosexuals.
How does Entamoeba histolytica cause disease?
- Entamoeba histolytica enters the body through oral route in cyst form.
- Cyst reaches to ileum and differentiate into trophozoites
- Trophozoites release enzymes and and invade the epithelium of large intestine or terminal ileum.
- Enzymes cause inflammation and necrosis of mucosa and submucosa and lesion reaches the muscularis mucosa and form characteristic flask shaped ulcers.
- From muscularis mucosa trophozoites may gain entrance into the portal blood stream.
- From portal vein it may reach to the liver to form a liver abscess or it may disseminate to the other organ of body such as lungs, heart diaphragm etc.
Clinical findings in amoebic dysentery and hepatic abscess.
Patient of acute infection of E. hystolytica presents with dysentery (bloody diarrhea), lower abdominal discomfort, flatulence and tenesmus. In case of chronic infections, diarrhea is usually occasional and symptoms like fever, fatigue and weight loss are also present. In case of liver abscess, fever, pain in the right upper quadrant of abdomen, weight loss, enlarged and tender liver are usual symptoms.
Diagnosis of E. histolytica infection
Diagnosis of E. histolytica can be made with the help of stool examination. Formed or fluid stools may reveal E. histolytica trophozoites or cysts. If stool examination comes negative then serological test are done in order to help in making the diagnosis. Biopsy from an ulcer during colonoscopy may also reveal trophozoites during microscopic examination. Liver abscess aspirates may also show trophozoites under the microscope. In chest X-ray the right dome of diaphragm may appear at higher than usual level due to presence of abscess under it. The commonest site of liver abscess is the superior surface of liver. CT scan, MRI, abdominal ultrasound may also be needed on individual basis. Liver function tests and blood complete profile may show increased level of liver enzymes and white blood cells respectively.
Treatment of Entamoebic dysentery or liver abscess.
The usual drug of choice in amoebic dysentery and liver abscess is metronidazole. Iodoqunol, tinidazole, diloxanide furoate and paromomycin are also used. Paromomycin and diloxanide furoate has the ability to kill the organism in the intestinal lumen, while metronidazole kill the organism in the solid tissue. So treatment with metronidazole should be followed by a 10-day course of paromomycin or diloxanide furoate to kill the residual organisms that are present in the intestinal lumen which may trigger another episode of dysentery or hepatic abscess in future.